Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code-division multiple access (CDMA) systems, time-division multiple access (TDMA) systems, frequency-division multiple access (FDMA) systems, and orthogonal frequency-division multiple access (OFDMA) systems.
By way of example, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple mobile devices. A base station may communicate with mobile devices on downlink channels (e.g., for transmissions from a base station to a mobile device) and uplink channels (e.g., for transmissions from a mobile device to a base station).
Some modes of communication may enable communications with a mobile device over different radio frequency spectrum bands (e.g., a licensed radio frequency spectrum band or an unlicensed radio frequency spectrum band) of a cellular network. With increasing data traffic in cellular networks, the offloading of at least some data traffic to an unlicensed radio frequency spectrum band may provide a cellular operator with opportunities for enhanced data transmission capacity. Prior to gaining access to and transmitting data over the unlicensed radio frequency spectrum band, a transmitting apparatus may, in some examples, perform a listen before talk (LBT) procedure to gain access to the unlicensed radio frequency spectrum band. An LBT procedure may include performing a clear channel assessment (CCA) to determine whether a particular channel of the unlicensed radio frequency spectrum band is available. When it is determined that a channel is not available, a CCA may be performed for the channel again at a later time.
Some transmissions of a transmitting apparatus (e.g., a base station or mobile device) may be made without performing a CCA. These CCA exempt transmissions (CETs) may include discovery signals (e.g., synchronization signals or reference signals), system information, or configuration information. If a mobile device fails to receive, from a base station, the CETs associated with a channel of an unlicensed radio frequency spectrum band, the mobile device may be unable to receive or transmit data over the channel of the unlicensed radio frequency spectrum band.